If you had gasoline contamination at as little as 5%, that could account for the explosion.
The concentration of vapors above a mixture of volatile liquids is controlled by Raoult's law, which states that the concentration of a substance in the vapor phase equals the concentration in the liquid phase times the vapor pressure of the substance. Low vapor pressure mixtures like gasoline quickly come to dominate the situation.
A flash point, if you can get it, wuld be useful. So would GC-mass spec.
John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[
www.firescientist.com]